Cordless power tool having multi-speed transmission and constant speed in light torque range

Abstract

A cordless power tool ( 300 ) has a transmission ( 12 ) having multiple speed ranges In the light torque range, the speed of a motor ( 8 ) of the cordless power tool is held constant to hold an output speed of the transmission ( 12 ) constant until motor power reaches a predetermined percentage of the maximum watts out of the motor.

What is claimed is: 
     
       1. A cordless power tool, comprising: a housing; a motor disposed in the housing; a gear case disposed in the housing that couples the motor to an output member, the gear case including multi-speed transmission having a plurality of speed ranges including a high speed/light torque range; a controller coupled to the motor that controls the speed of the motor, the controller being configured to drive the motor with a pulse width modulated signal and vary a duty cycle of the pulse width modulated signal to control the speed of the motor; the controller varying the power output of the motor to maintain an output speed of the motor constant when the multi-speed transmission is in the high speed/light torque range until motor power reaches a predetermined percentage of maximum watts out of the motor and then allowing the output speed to follow a natural torque-speed curve of the motor. 
     
     
       2. The apparatus of  claim 1  wherein the power tool is a variable speed power tool having a trigger, the controller allowing the motor speed to follow the natural torque-speed curve of the motor when the trigger is in less than a full on position when the multi-speed transmission is in the high speed/light torque range. 
     
     
       3. The apparatus of  claim 1  wherein the controller varies the duty cycle of the pulse width modulated signal from seventy percent on to one hundred percent on when maintaining the speed of the motor constant. 
     
     
       4. The apparatus of  claim 3  wherein the power tool is a variable speed drill having a trigger, the controller allowing the motor speed to follow the natural torque-speed curve of the motor when the trigger is in less than a full on position when the multi-speed transmission is in the high speed/light torque range. 
     
     
       5. The apparatus of  claim 1  wherein the predetermined percentage of maximum watts out of the motor is seventy-five percent. 
     
     
       6. The apparatus of  claim 1  wherein the motor has a maximum no-load speed that is at least fifty-percent higher than the speed at which the motor speed is kept constant when the controller is maintaining the speed of the motor constant when the multi-speed transmission is in the high speed/light torque range. 
     
     
       7. The apparatus of  claim 1 , wherein the controller receives a signal indicative of whether the multi-speed transmission is in the high speed/light torque range and controls the output speed based on said signal. 
     
     
       8. A variable-speed cordless power tool, comprising: a housing; a motor disposed in the housing; a variable-speed trigger; a gear case disposed in the housing that couples the motor to an output member, the gear case including multi-speed transmission having a plurality of speed ranges, including a high speed/light torque range; a controller coupled to the motor that controls the speed of the motor based on an input from the variable-speed trigger; wherein, for a given displacement position of the trigger, the controller maintains an output speed of the motor constant when the multi-speed transmission is in the high speed/light torque range until motor power reaches a predetermined percentage of maximum watts out of the motor and then allows the output speed to follow a natural torque-speed curve of the motor. 
     
     
       9. The apparatus of  claim 8 , the controller allowing the motor speed to follow the natural torque-speed curve of the motor when the trigger is in less than a full on position when the multi-speed transmission is in the high speed/light torque range. 
     
     
       10. The apparatus of  claim 8  wherein the controller drives the motor with a pulse width modulated signal and varies a duty cycle of the pulse width modulated signal to control the speed of the motor. 
     
     
       11. The apparatus of  claim 10  wherein the controller varies the duty cycle of the pulse width modulated signal from seventy percent on to one hundred percent on when maintaining the speed of the motor constant. 
     
     
       12. The apparatus of  claim 11 , the controller allowing the motor speed to follow the natural torque-speed curve of the motor when the trigger is in less than a full on position when the multi-speed transmission is in the high speed/light torque range. 
     
     
       13. The apparatus of  claim 8  wherein the predetermined percentage of maximum watts out of the motor is seventy-five percent. 
     
     
       14. The apparatus of  claim 8  wherein the motor has a maximum no-load speed that is at least fifty-percent higher than the speed at which the motor speed is kept constant when the controller is maintaining the speed of the motor constant when the multi-speed transmission is in the high speed/light torque range.